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(CNN) -- Women in the United States are waiting longer to have children, health statistics show, presenting challenges for doctors and patients alike.

More than 586,000 babies were born to women over age 35 in 2004, according to preliminary figures from the Centers for Disease Control and Prevention.

That's just over 14 percent of births that year, up from about 5 percent 15 years ago, said Mary E. D'Alta, the chair of Columbia University's Department of Obstetrics and Gynecology.

The average age of women when they gave birth for the first time was at a record high of 25.2 in 2003, the CDC said. That age has increased by nearly four years since 1970.

Janet Silber, a North Carolina freelance writer, said she and her husband didn't really start thinking about children until after she was 40.

"We were both really involved with our careers. It wasn't until I closed my company. I was a licensed general-contractor ... I all the sudden felt 'Oh no, I forgot to do this.' "

She said she was concerned they might have trouble getting pregnant and asked the doctor to run "the full workup, right now, because we don't have a lot of time here."

Her doctor told them not to worry unless they weren't able to conceive in six months.

"We didn't have any problems with fertility fortunately, and that's something I'm really grateful for, because I have a lot of friends who did," Silber said.

Machelle Seibel, a professor of obstetrics and gynecology at the University of Massachusetts, said many women who wait are surprised to have fertility problems.

"As you get people who are delaying child bearing in order to pursue their careers ... they're accustomed to working hard and being able to accomplish goals that they put their mind to," he said. "I think many ... haven't been adequately informed that they're going to have difficulty having a kid."

Women over 35 are at a higher risk of developing diabetes or high blood pressure during pregnancy and are more likely to be overweight or have other health risks before they conceive, D'Alta said.

They also face higher risks of miscarriage, multiple births, having a baby with genetic disorders, according to the March of Dimes.

"One thing we do is to give more careful prenatal care to women who have risk factors, and the goal of prenatal care is to identify risk and to minimize it," D'Alta said.

She said doctors' ability to diagnose problems before birth has improved dramatically.

"Our technology is getting better, our ultrasound is getting better, our doctors and care providers are becoming much more experienced at ultrasound, and in certain situations we can use MRI (magnetic resonance imaging) before the baby is born to give us more accurate information," she said.

Raul Artal, the head of obstetrics and gynecology at St. Louis University School of Medicine, says doctors also have started to put more emphasis on fitness for all pregnant women, but especially those with diabetes complications.

"Pregnancy is no longer a state of confinement. In the old days women were advised to rest as much as possible and to eat for two," Artal said. "Women were discouraged from engaging in physical activity during their pregnancy."

Silber said her pregnancy was very normal and even "kind of boring."

"Of course, all my relatives were really worried," she said. "Every time I passed (a milestone) like the first trimester, they were like 'Thank God you didn't have a miscarriage.' "

Doctors had to induce labor and eventually delivered her daughter Rebecca by cesarean section after finding that her umbilical cord was wrapped around her neck.

"That wasn't age related, but it was a problem," she said. "My biggest concern up until then was I hope I don't get any larger or I'm going to have to get a seatbelt extender."

Birth more controlled

D'Arta said that in general, older women tend to want to have more control of their birth experience.

"They're more frequently asking to be induced because they're concerned about not having their own doctors around for their delivery, and some women are also asking for cesarean deliveries," D'Arta said.

The number of cesarean deliveries has reached an all-time high. In 2004, 29.1 percent of babies, or almost 1.2 million, were born by cesarean, according to the CDC. That's up from 27.5 percent in 2003, which also was a record.

D'Arta was a member of a National Institutes of Health panel that recently called for more study into the potential risks and benefits of elective cesarean sections.

She said it was not clear how many cesareans are done at the woman's request, but said that 5 percent was a reasonable estimate.

'What's important is different'

Silber said that once she had her baby she found herself spending a lot more time with young mothers.

"They were very nice, and the kids were all the same age, but I think the difference is your values are different, what's important is different," she said. "I kind of grew up in the hippie era, and they were maybe children of people who grew up in the hippie era."

She said it was nice when she met women who were her age and shared "a common reference point."

Silber said her experience of raising a child did not seem much different from that of younger moms.

"When you're my age now, a 50-year-old with a 10-year-old, rather than being 40 with a 10-year-old that's where you begin to notice a difference because you've got the knee problems or the back problems," she said. "You just can't go out and play basketball with them all afternoon like you want to. That's the hard part."